Role of microbial iron reduction in arsenic metabolism from soil particle size fractions in simulated human gastrointestinal tract

Gut microbiota provides protection against arsenic (As) induced toxicity, and As metabolism is considered an important part of risk assessment associated with soil As exposures. However, little is known about microbial iron(III) reduction and its role in metabolism of soil-bound As in the human gut. Here, we determined the dissolution and transformation of As and Fe from incidental ingestion of contaminated soils as a function of particle size (<250 μm, 100-250 μm, 50-100 μm and < 50 μm). Colon incubation with human gut microbiota yielded a high degree of As reduction and methylation of up to 53.4 and 0.074 μg/(log CFU/mL)/hr, respectively; methylation percentage increased with increasing soil organic matter and decreasing soil pore size. We also found significant microbial Fe(III) reduction and high levels of Fe(II) (48 %-100 % of total soluble Fe) may promote the capacity of As methylation. Although no statistical change in Fe phases was observed with low Fe dissolution and high molar Fe/As ratios, higher As bioaccessibility of colon phase (avg. 29.4 %) was mainly contributed from reductive dissolution of As(V)-bearing Fe(III) (oxy)hydroxides. Our results suggest that As mobility and biotransformation by human gut microbiota (carrying arrA and arsC genes) are strongly controlled by microbial Fe(III) reduction coupled with soil particle size. This will expand our knowledge on oral bioavailability of soil As and health risks from exposure to contaminated soils.

Arsenic in Rice Bran Products: In Vitro Oral Bioaccessibility, Arsenic Transformation by Human Gut Microbiota, and Human Health Risk Assessment

Despite rice consumption, rice bran as a byproduct of rice milling contains higher arsenic (As). The present study evaluated the metabolic potency of in vitro cultured human colon microbiota toward As from five rice bran products with 0.471-1.491 mg of As/kg. Arsenic bioaccessibility ranged from 52.8 to 78.8% in the gastric phase, and a 1.2-fold increase (66.0-95.8%) was observed upon the small intestinal phase. Subsequently, a significant decline of As bioaccessibility (11.3-63.6%) and a high methylation percentage of 18.5-79.8% were found in the colon phase. The predominant As species in the solid phase was always As(V) (49.6-63.4%), and As-thiolate complexes increased by 10% at the end of colon incubation. Human gut microbiota could induce As bioaccessibility lowering and As transformation in rice bran, which illustrated the importance of food-bound As metabolism in the human body. This will result in a better understanding of health implications associated with As exposures.